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Signatures of Dark Matter in Cosmic Antimatter Fluxes: Results from the PAMELA Experiment

This presentation discusses the results from the PAMELA experiment on the detection of dark matter signatures in cosmic antimatter fluxes. It focuses on the antimatter component of cosmic rays and explores various phenomena such as evaporation of primordial black holes, anti-nucleosynthesis, and dark matter annihilation.

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Signatures of Dark Matter in Cosmic Antimatter Fluxes: Results from the PAMELA Experiment

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  1. Signatures of dark matter in cosmic antimatter fluxes: results from the PAMELA experiment Roberta Sparvoli University of Rome “Tor Vergata” and INFN Rome, Italy On behalf of the PAMELA collaboration Roberta SparvoliTEVO8

  2. PAMELAPayload for Antimatter/Matter Exploration and Light-nuclei Astrophysics  Direct detection of CRs in space  Main focus on antimatter component Roberta SparvoliTEVO8

  3. Why CR antimatter? Evaporation of primordial black holes Anti-nucleosyntesis First historical measurements of p-bar/p ratio WIMP dark-matter annihilation in the galactic halo Background: CR interaction with ISM CR + ISM  p-bar + … Roberta SparvoliTEVO8

  4. Charge-dependent solar modulation Solar polarity reversal 1999/2000 Asaoka Y. Et al. 2002 Positron excess? ? ? ¯ + CR + ISM  p-bar + … kinematic treshold: 5.6 GeV for the reaction CR antimatter Present status Positrons Antiprotons ___ Moskalenko & Strong 1998 CR + ISM  p± + x m± + x  e± + x CR + ISM  p0 + x gg e± Roberta SparvoliTEVO8

  5. PAMELA detectors Main requirements  high-sensitivity antiparticle identification and precise momentum measure + - • Time-Of-Flight • plastic scintillators + PMT: • Trigger • Albedo rejection; • Mass identification up to 1 GeV; • - Charge identification from dE/dX. • Electromagnetic calorimeter • W/Si sampling (16.3 X0, 0.6 λI) • Discrimination e+ / p, anti-p / e- • (shower topology) • Direct E measurement for e- • Neutron detector • plastic scintillators + PMT: • High-energy e/h discrimination GF: 21.5 cm2 sr Mass: 470 kg Size: 130x70x70 cm3 Power Budget: 360W • Spectrometer • microstrip silicon tracking system+ permanent magnet • It provides: • - Magnetic rigidity R = pc/Ze • Charge sign • Charge value from dE/dx Roberta SparvoliTEVO8

  6. Principle of operation Track reconstruction • Measured @ground with protons of known momentum •  MDR~1TV • Cross-check in flight with protons (alignment) and electrons (energy from calorimeter) Iterative c2 minimization as a function of track state-vector components a Magnetic deflection |η| = 1/R R = pc/Ze magnetic rigidity sR/R = sh/h Maximum Detectable Rigidity (MDR) def: @ R=MDR sR/R=1 MDR = 1/sh Roberta SparvoliTEVO8

  7. track average 4He B,C 3He Be d (saturation) p Li e± 1st plane Principle of operation Z measurement Bethe Bloch ionization energy-loss of heavy (M>>me) charged particles Roberta SparvoliTEVO8

  8. Principle of operation Velocity measurement • Particle identification @ low energy • Identify albedo (up-ward going particles b < 0 ) •  NB! They mimic antimatter! Roberta SparvoliTEVO8

  9. Principle of operation Electron/hadron separation • Interaction topology • e/h separation • Energy measurement of electrons and positrons • (~full shower containment) hadron (19GV) electron (17GV) + NEUTRONS!! Roberta SparvoliTEVO8

  10. 1 GV 5 GV Antiproton identification -1  Z  +1 p (+ e+) p e-(+ p-bar) proton-consistency cuts (dE/dx vs R and b vs R) “spillover” p p-bar electron-rejection cuts based on calorimeter-pattern topology The main difficulty for the antiproton measurement is the spillover proton bkg @ high energy:  finite deflection resolution of the spectrometer  p/p-bar ~ 104 !! Roberta SparvoliTEVO8

  11. 10 GV 50 GV High-energy antiproton selection p p-bar “spillover” p R < MDR/10 MDR = 1/sh (evaluated event-by-event by the fitting routine) • MDR depends on: • number and distribution of fitted points along the trajectory • spatial resolution of the single position measurements • magnetic field intensity along the trajectory Roberta SparvoliTEVO8

  12. Antiproton-to-proton ratio1 – 100 GeV Submitted to PRL on 13/09/2008 Roberta SparvoliTEVO8

  13. Antiproton-to-proton ratio (statistical errors only) Roberta SparvoliTEVO8

  14. Positron identification • The main difficulty for the positron measurement is the interacting-proton bkg: • fluctuations in hadronic shower development  p0 ggmight mimic pure em showers • proton spectrum harder than positron  p/e+ increase for increasing energy Energy-rigidity match e- ( e+ )  e  h p-bar p Roberta SparvoliTEVO8

  15. Proton background suppression Fraction of charge released along the calorimeter track (left, hit, right) Z=-1 e- Rigidity: 20-30 GV p-bar (non-int) p-bar (int) NB! Z=+1 p (non-int) (e+) p (int) Roberta SparvoliTEVO8

  16. Proton background suppression Fraction of charge released along the calorimeter track (left, hit, right) Z=-1 e- Rigidity: 20-30 GV + Constraints on: p-bar Energy-rigidity match Z=+1 e+ p Roberta SparvoliTEVO8

  17. Proton background suppression Fraction of charge released along the calorimeter track (left, hit, right) Z=-1 e- Rigidity: 20-30 GV + Constraints on: Energy-momentum match Shower starting-point Z=+1 Longitudinal profile e+ p Roberta SparvoliTEVO8

  18. Tests on the positron selection Fraction of charge released along the calorimeter track (left, hit, right) Flight data: rigidity: 20-30 GV Test beam data Momentum: 50 GeV/c e- e- e- p e+ p e+ p • Energy-rigidity match • Starting point of shower Roberta SparvoliTEVO8

  19. Tests on the positron selection Rigidity: 20-30 GV Fraction of charge released along the calorimeter track (left, hit, right) Neutrons detected by ND e- e- p e+ e+ p • Energy-rigidity match • Starting point of shower Roberta SparvoliTEVO8

  20. Positron charge ratio Preliminary!! statistical errors only energy in the spectrometer ___ Moskalenko & Strong 1998 Currently under embargo Roberta SparvoliTEVO8

  21. ¯ ¯ Drift Model --- Clem 1995 --- Bibier 1999 + + A > 0 Positive particles A < 0 Clem J. & Evenson 2007 Charge dependent solar modulation Preliminary!! Roberta SparvoliTEVO8

  22. High-energy positrons Pulsars LSP (Neutralino) annihilation statistical errors only energy in the spectrometer (Strong-Moskalenko-Ptuskin 2007) ___ Moskalenko & Strong 1998 Contribution from pulsars ? Contribution from WIMP annihilation in the galactic halo Roberta SparvoliTEVO8

  23. Galactic H and He spectra Preliminary!! (statistical errors only) Very high statistics over a wide energy range  Precise measurement of spectral shape  Possibility to study time variations and transient phenomena Roberta SparvoliTEVO8

  24. Power-law fit: ~ E-g g ~ 2.76 LBM NB! still large discrepancies among different primary flux measurements Proton flux * E2.75 Preliminary!! (statistical errors only) • Proton of primary origin • Diffusive shock-wave acceleration in SNRs • Local spectrum: • injection spectrum  galactic propagation • Local primary spectral shape: • study of particle acceleration mechanism Roberta SparvoliTEVO8

  25. LBM Secondary nuclei Preliminary!! (statistical errors only) PAMELA • B nuclei of secondary origin: • CNO + ISM  B + … • Local secondary/primary ratio sensitive to average amount of traversed matter (lesc) from the source to the solar system • Local secondary abundance: • study of galactic CR propagation • (B/C used for tuning of propagation models) Roberta SparvoliTEVO8

  26. Conclusions • PAMELA is taking data since July 2006 (lot of fun analyzing the data!) • Presented preliminary results from ~600 days of data: • Antiproton charge ratio (~ 100 MeV ÷ 100 GeV) • no evident deviations from secondary expectations • more high energy data to come (up to ~150 GeV) • Positron charge ratio (~ 400 MeV ÷ 10 GeV) • indicates charge dependent modulation effects • more data to come at lower and higher energies (up to ~ 200 GeV) • Galactic primary proton spectra • primary spectra up to Z=8 to come • Galactic secondary-to-primary ratio (B/C) • abundance of other secondary elements (Li,Be) and isotopes (d,3He) to come  PAMELA is providing significant experimental results for dark matter searches and for understanding CR origin and propagation Please attend the talk of prof. P. Picozza for more details (Friday, 10:45) Roberta SparvoliTEVO8

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